1. Field of the invention
The invention relates to the field of polymerization in a gaseous phase with the use of heterogeneous catalysis. It more particularly relates to a process and a reactor for carrying out such polymerization reactions.
Many monomers may be polymerized in a gaseous phase with heterogeneous catalysis. This is in particular the case of unsaturated ethylenically insaturated hydrocarbons and in particular olefins. For the homopolymerization as well as the copolymerization reactions, the monomer is put in contact in the gaseous form with a catalyst dispersed in the solid form, so that the catalysis is heterogeneous.
In the course of the polymerization, the polymer is formed around the catalyst particles and, if the process is carried out at a temperature lower than the melting point of the polymer, a polymer powder is finally obtained. The mean particle size of the polymer powder obtained is usually larger than that of the catalyst powder.
Polymerizations in a gaseous phase are mainly carried out in reactors having a fluid bed or an agitated bed.
The apparatus employing fluid beds are not devoid of drawbacks. Indeed, it is known that the fluidization of the powders is a delicate operation which requires precise particle size distributions. The fine particles have a tendency to be carried out of the bed while the largest particles have a tendency to settle on the gas distribution grate of the reactor. The conditions of gas flow for the fluidization and the thermal exchange inside the reactors are intimately related so that the operation of the latter is not flexible. In any case, the speed of circulation of the gases must be equal to or higher than the minimum speed of fluidization. Further, the starting-up of a fluid bed reactor requires the prior charging of the reactor with a certain quantity of particulate material, essentially the polymer powder. Without this prior charging, there is not enough catalyst powder to create the fluidized bed.
Conventional apparatus for carrying out polymerization reactions with a heterogeneous catalyst are agitated bed reactors. These types of reactors may be in several forms. A good number of them are vertical; it for example concerns cylinders having a rounded bottom provided with an agitator. Horizontal reactors also exist. In any case, an initial charge of polymer powder must be used. lt is difficult to achieve a well-homogeneous agitation within the reactor, so that there are increased risks of agglomeration of the growing polymer powder in the poorly agitated zones. The agitator system comprises blades and counter-blades, which creates dead zones. The agitation obtained is not uniform and homogeneous.
This problem is particularly felt when carrying out prepolymerization reactions for the purpose of controlling the particle sizes of the growing powders and controlling the extremely high initial activities of certain types of catalysts. With conventional fluid bed reactors and agitated bed reactors, the formation of hot points and agglomerates is encountered in the dead or poorly agitated zones, which renders the product heterogeneous and unsuitable for use in a main polymerization reactor.
An object of the invention is to provide an improved polymerization process in a gaseous phase and heterogeneous catalysis, in a polymerization reactor which avoids the drawbacks of the techniques of the prior art whether they concern a fluid bed or an agitated bed. In respect of the present invention, and in the ensuing specification, polymerization is intended to mean all stages of the polymerization reaction permitting the obtainment of a dry powder covering very wide particle size ranges.
Thus the described polymerization process may serve to form a prepolymer which is injectable into a second polymerization reactor. In particular, this polymerization reactor may be in a gaseous phase of the same type (agitated bed) or of another type (for example, fluid bed). The described polymerization process may also serve to form the final polymer directly in a homogeneous powdered form devoid of agglomerates.
According to the invention, the agitation is perfectly homogeneous and leaves no dead zones, which results in an excellent dispersion of the catalyst, of the growing polymer powder and of the gases of the reactor. According to the invention, it is possible to be substantially independent of the particle size of the catalysts and polymer powder, and consequently be able to employ a much wider range of solid catalysts. The invention enables the polymerization to be started up with a very small, and even zero, charge of powder if necessary.
In its most general form, the invention provides a polymerization process in a gaseous phase with the use of heterogeneous catalysis by putting at least one monomer, which is gaseous under the conditions of the reaction, in contact with a solid catalyst in an agitated polymerization zone, wherein there is employed a polymerization zone defined by a spherical wall and the agitation is produced by means of a turbine unit having blades and driven in rotation, said blades extending along said wall in the 10 to 60% of its surface, the particles of the catalyst and the growing polymer powder being driven by centrifugal force in at least a part of the spherical wall and dropping into the central part of the spherical zone, thereby ensuring an through and uniform mixing without dead zones.
The invention is essentially applicable to polymerizations in a gaseous phase with the use of heterogeneous catalysis. This definition implies that it is possible to emply a large variety of monomers capable of being put into the gaseous form under the polymerization conditions prevailing within the reactor. Liquefiable monomers may also be introduced by injecting them under pressure in the polymerization zone. In the latter, the liquefiable monomers vaporize so that the monomer or the comonomer is in a gaseous phase in the polymerization reaction. The gaseous atmosphere of the reactor may possibly contain gases which are inert concerning the polymerization reaction and gases acting on the transfer reactions. High proportions of inert gases are employed with low partial pressures of monomer(s), in particular when the reactor operates for prepolymerization, so as to control the initial rates of the catalysts and the thermal exchanges.
The form of the spherical polymerization zone employed in accordance with the invention permits operating within a wide range of pressures and temperatures. It is possible to operate just as well in a vacuum or under high pressure. Possible ranges of pressure range from values lower than atmospheric pressure up to 500 atmospheres or more, the preferred range being between about 1 and 80 atmospheres. The particular conditions to be chosen will of course depend on the nature of the monomer or monomers to be polymerized.
The temperature conditions are note critical either. It is just as possible to operate below or above room temperature. Generally, the range of suitable temperatures ranges from normal temperature to 250.degree. C. or more and is preferably between room temperature and about 150.degree. C. Here again, it is the nature of the monomer or monomers to be polymerized or of the polymer obtained which will enable a person skilled in the art to choose the most appropriate particular temperature conditions, it being possible to carry out the invention at any usual temperature of polymerization reactions in a gaseous phase with heterogeneous catalysis.
The preferred monomers for the polymerization according to the invention are ethelynically unsaturated hydrocarbons. The new process permits in particular the polymerization of olefins and the copolymerization of olefins among each other to obtain polyolefins of variable density and structure. Apart from ethylene and propylene, which represent particularly preferred monomers, it is possible to employ various .alpha.-olefins comprising preferably 3 to 18 carbon atoms and, better still, 3 to 8 carbon atoms, including 1-butene, 1-pentene, 4-methyl-1-pentene, 1-heptene, 1-hexene, 1-octene and mixtures thereof. The copolymerization reactions which are preferred are those which employ the copolymerization of ethylene and propylene, and the copolymerization of ethylene and/or olefins of 3 to 18 carbon atoms, these being straight chain or branched chain. By way of examples, there may be mentioned 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene and mixtures thereof. Within the meaning of the present description, the expression "olefins" also covers the di-olefins and conjugated di-olefins. The dienes, such as butadiene, isoprene, 1, 4-hexadiene 1, 5-hexadiene, vinylnorbornene, ethylidenenorbornene, and dicyclopentadiene may be used to advantage as co-monomers and specially as ter-monomers in a polymerization reaction according to the invention.
The modes of introduction of the monomer(s) to be polymerized in the spherical polymerization zone will depend on the form in which these monomers are available under the conditions of temperature and pressure of the storage.
It may be of interest to introduce the monomer(s) in such manner that the pressure remains substantially constant within the polymerization zone. In the case of gaseous monomers, such as ethylene, the introduction may be controlled by known means, for example by a pressure controller provided in the monomer(s) supply means.
The liquefiable monomers, for example the 1-butene, may be continuously injected into the polymerization zone by means known to those skilled in the art, so that the pressure within the polymerization zone remains substantially constant. In small-scale plants a diaphragm pump is suitable for this purpose.
When it is desired to effect copolymerization reactions, it is advantageous to supply the monomers to be copolymerized in relatively well-defined proportions. For this purpose it is sufficient to put the means for introducing a monomer under the control of those provided for the other monomer(s) so as to achieve the desired ratio. In the case of a mixture of gaseous ethylene and liquid 1-butene, the injection pump of the 1-butene may then be a variable output pump controlled by the signal delivered by the flow-meter furnishing information concerning the quantity of gaseous ethylene introduced. In this way, the butene/ethylene ratio is maintained constant automatically. It is also possible to emply a gas-analyzer for controlling the various flows.
All the foregoing indications concerning the nature of the monomers or co-monomers to be employed and the means for introducing them in the polymerization zone are known to those skilled in the art and have no need to be described in more detail.
The polymerization reactions are achieved with a solid catalyst preferably presented in the granular or particulate form. These catalyst systems are also wellknown to those skilled in the art. In heterogeneous catalysis, in particular for the olefins, use is generally made of the supported catalysts of the Ziegler-Natta type or supported chromium with or without an appropriate co-catalyst. In catalysis of the Ziegler type, catalysts having a very high activity are preferably employed whose most conventional composition comprises a titanium compound, a magnesium compound, possibly an electron-donor organic compound and possibly a porous support of the metal oxide type. These catalysts are generally employed in the presence of co-catalysts.
Among the titanium compounds there may be mentioned among others, titanium chlorides (TiCl.sub.3,TiCl.sub.4) and chlorotitanates.
The magnesium compound is generally a magnesium halide, preferably MgCl.sub.2.
In certain cases, complexing agents may be employed owing to their electron donor power.
Belonging to these compounds are the Lewis bases which include compounds such as the alkyl esters obtained from aromatic or aliphatic carboxylic acids, aliphatic or cyclic ethers, and ketones. The preferred electron donors comprise methyl formiate, methyl paratoluate, ethyl or butyl acetate, ethyl ether, tetrahydrofurane dioxane, acetone, and methyl isobutyl ketone.
The titanium, Mg and electron donor compounds may be distributed on a porous support of the type silica gel and silica/alumina for example.
In this case it concerns gels having a large surface (300m.sup.2 /g) and a large porous volume, for example of the order of 1.7 cm.sup.3 /g. A silica 952 of the firm GRACE satisfies this type of specification.
The co-catalysts are of the type Al(R).sub.a X.sub.b H.sub.c with X=Cl or OR; R is a saturated alkyl radical of C.sub.1 to C.sub.14 with a +b +c =3. Such activators comprise for example Al(C.sub.2 H.sub.5).sub.3, Al(C.sub.2 H.sub.5).sub.2 Cl, Al (i-C.sub.4 H.sub.9).sub.3, A1.sub.2 (C.sub.2 H.sub.5).sub.3 Cl.sub.3, Al( iC.sub.4 H.sub.9)H.sub.2, Al(C.sub.6 H.sub.13).sub.3, Al(C.sub.8 H.sub.17).sub.3, Al(C.sub.2 H.sub.5)H.sub.2 and Al(C.sub.2 H.sub.5).sub.2 (O C.sub.2 H.sub.5).
In the catalysts of the Ziegler type, the titanium may be replaced by other transition metals such as zirconium or vanadium.
With catalysts of the supported chromium type, use is made of the chromium spread over a metal oxide porous support; if desired, other compounds may be spread over the porous support such as titanium coumpounds. A fluorination of the catalyst may also be effected.
Before being used in the polymerization, these catalysts are subjected to a heat treatment at high temperature in an oxidizing and anhydrous atmosphere.
Among the chromium compounds which may be used there may be mentioned chromium acetylacetonate, organic chromates, chromium acetates and chromium oxide (CrO.sub.3).
Among the suitable supports, there may be mentioned the silica gels or alumina silica gels (for example silica GRACE 952). The other titanium compounds may be alkyl titanates or chlorotitanates. The fluorination can be carried out by thermal decomposition of salts giving off hydrofluoric acid. Salts such as (NH.sub.4).sub.2 SiF.sub.6 or (NH.sub.4).sub.2 TiF.sub.6 are good fluorination agents.
The heat treatment may be carried out under dry air at temperatures of the order of 600.degree. to 800.degree. C.
The activated catalysts obtained may if desired be employed in the presence of co-catalysts of the same type as those employed in Ziegler catalysis.
The two catalytic systems described may be advantageously employed for the polymerization of the ethylene or the copolymerization of the ethylene with .alpha.-olefins.
The Ziegler catalysts are also employed in the polymerization of the propylene and 1-butene to obtain products having a high isotacticity index.
As is usual in the polymerization reactions of olefins, they must be carried out away from air and humidity and consequently the polymerization zone has all traces of humidity advantageously removed therefrom by flushing with a gas such as hydrogen, nitrogen or argon or by contacting one of the components of the catalyst which is capable of cleaning the polymerization zone, which is the case for example of the alkylaluminum compounds, co-catalysts in the Ziegler catalysis.
The invention combines the spherical shape of the polymerization zone and the nature of the agitating means which gives particularly advantageous and surprising results in the polymerization reactions.
The agitating means contained in the spherical polymerization zone comprise mainly a turbine unit having shaped blades. The blades of the turbine are substantially very close to the spherical wall defining the polymerization zone. In practice, it has been found that clearances of 1 to 50 mm were suitable. The turbine blades cooperate with the spherical wall on about 10 to 60% of its surface. It has been found that blades which would extend over an excessively small zone, less than about 10% of the spherical surface of the polymerization zone, would not produce a sufficiently uniform and homogeneous agitation to obtain the results of the invention. On the other hand, it is unnecessary, and even disadvantageous, that the blades extend beyond a limit representing about 60% of the spherical surface. For a practical construction, the turbine has blades of such length that the diameter of the circle described by their ends is at least equal to one third of the diameter of the sphere. The blades may pass alongside the spherical wall until they reach an equatorial plane in which case the diameter the circle generated by their ends is equal to at least the diameter of the sphere minus the clearance. These blades may even extend slightly beyond the equatorial plane normal to the axis of rotation.
Advantageously, the turbine unit comprises two to eight blades an preferably three blades symmetrically arranged around the agitation axis and having such profiles as to ensure an thorough mixing.
In addition, but without this being essential, the agitating means may comprise, in the part of the spherical polymerization zone unoccupied by the turbine unit, an additional scraper system comprising one or more elements driven in rotation. These elements are preferably filiform blades having such profiles as to avoid disturbing the travelling of the gases.
Optionally, the agitating means may further comprise one or more turbines termed dilacerating turbines, the function of which, if required, is to reduce the size of the agglomerates liable to be formed during the polymerization. These dilacerating turbines are located in the zone above the circle generated by the end of the blades of the main agitator turbine.
Owing to the spherical shape of the polymerization zone, the mixing of the gases and powders inside the zone is considerable. It is thought that this is due to the fact that the centrifugal force decreases as the polymer powder rises above the equatorial plane of the reactor. In the course of this rise of the powder the effect of gravity prevails beyond a certain height and the grains fall toward the centre of the sphere. This mixing is achieved without need to employ counterblades which create dead or badly stirred zones. The grains are well-dispersed irrespective of their diameter. The additional scraper system, which has such profile as to avoid disturbing the passage of the gases, permits the avoidance of any possible electrostatic or other agglomeration of the polymer powder being formed.
The combination of the spherical shape of the polymerization zone and the manner of agitating also permits the obtainment of excellent coefficients of transfer with the wall and intimate contact between the fluids introduced in the reactor and the growing polymer powder while maintaining a good fluidization of the powders without formation of any unagitated zone. Owing to this combination of the spherical shape and the agitating means, there is obtained a perfect dispersion of the catalyst, of the growing polymer powder and of the gaseous reaction mixture. Thus the polymerization is carried out always under good conditions, irrespective of the charge and the grain size of the polymer present in the polymerization zone, it being possible that this charge be zero or almost zero when starting up.
Further, the grain size of the catalysts and the polymer powders becomes unimportant. Thus it is possible to employ according to the invention a much wider range of solid catalysts.
According to the invention, the polymerization temperature is also controlled. The means employed for this purpose are not critical and may be of a very varied type. According to one embodiment, they may comprise a jacket with inlets and outlets for the circulation of the controlling fluid. In the case of the invention, the jacket conforms to the shape of the wall defining the polymerization zone and therefore has a generally spherical shape. It is provided with respective pipes for the circulation of the controlling fluid.
In this first manner of controlling the temperature, the temperature is controlled from the exterior. According to other controlling modes, which may be found to be preferable, the temperature is controlled inside the very polymerization zone, which is particularly advantageous in the present case owing to the spherical shape. For this purpose there may be injected into the polymerization zone a cooled gas, a compressed fluid which cools by expansion, or a liquid which vaporizes under the conditions prevailing within the polymerization zone.
According to another aspect, the invention also concerns a gaseous phase polymerization reactor employing heterogeneous catalysis which comprises mainly means for introducing the monomer or monomers to be polymerized and the catalytic system, means for agitating the catalyst in solid particles and the polymer being formed, means for controlling the temperature and means for withdrawing the polymer obtained, said reactor being of an essentially spherical shape and the agitating means comprising mainly a turbine unit having shaped blades driven in rotation, said blades passing alongside the inner wall of the reactor on 10 to 60% of its surface.
As concerns the features of the agitating means for the reactor according to the invention reference will be made to what has already been indicated. The turbine unit is placed in the lower part of the reactor. The pivot of this unit is disposed in the region of the wall of the reactor at a point located in the lower part of the latter, but not necessarily at the lowest point of the sphere. The driving means for the turbine unit comprise a shaft which extends through the wall of the reactor in a sealed manner. The shaft may be vertical or have any other oblique position relative to the vertical, depending on the position of the pivot of the turbine unit. The shaft may be short, in particular in the case where the driving means are located in the immediate vicinity of the wall of the reactor, in which case the shaft merely extends through the wall and connects the driving means to the pivot of the turbine unit. In other cases, the shaft may extend through the reactor, for example if the driving means are located in the upper part of the reactor. In such cases, the shaft may be vertical, but this position is not critical.
If the additional scraper system is included, it is disposed in the upper part of the spherical reactor. It may be actuated by driving means relating thereto or by the same means as those employed for driving the turbine unit, which constitutes the main agitation means in the reactor according to the invention. If one or more dilacerating turbines are present, they are located in the part above the circle generated by the end of the poles of the main turbine.
As concerns the means controlling the temperature, reference will be made also to the foregoing description of the process according to the invention.
The reactor according to the invention also comprises means for drawing off the polymer obtained, for example at least one valve with a discharge pipe connected in the lower part of the spherical reactor.
The reactor should also be capable of being put in communication with the atmosphere. For this purpose, a pipe may be connected to the upper dome of the reactor.
Means are also provided for introducing renewed quantities of catalysts and possibly chain limitors such as hydrogen, in particular for continuous operation.
The reactor according to the invention lends itself perfectly well to a continuous operation. The polymer powder is taken off in the lower part of the reactor and a supply of the monomer(s) is introduced which corresponds to their consumption in the course of the polymerization reaction; the gaseous monomer is thus absorbed as the reaction proceeds. A device for introducing the catalysts enables the exothermic type of reaction to be maintained.
Note also that for certain needs, the reactor may be provided with a lock device for taking off samples of products in the course of the reaction. This lock may be employed in the form of a pipe provided with valves connected to any part of the reactor and capable of being flushed by an inert atmosphere.
An advantageous embodiment of a reactor of small size according to the invention comprises two semi-spheres assembled in their equatorial plane. The two semi-spheres are assembled by clamping means, for example cramps arranged around the periphery of the reactor in the equatorial plane of assembly. The semi-spheres may thus be easily disassembled in order to have access to the interior of the reactor.
The form of the industrial reactors according to the invention is not necessarily that of a complete integral sphere. According to the invention, the sole important characteristic is that the polymerization zone be defined by a spherical wall. But, however, various pipes, connections or openings may interrupt or modify the spherical shape of the reactor. Thus, for convenience of use and in particular cleaning, the reactor may comprise openings of sufficient size to permit access to the interior and, for example, allow the entry of a man in the case of large reactors.
In practice, the reactor of the invention is an integral part of a more complete plant comprising monomer supply circuits, inert gas and chain regulating gas supply circuits. In the case of gaseous monomers, these circuits comprise means for putting under pressure the monomer gas and establishing a constant introduction pressure, such as defined for example by a pressure regulator. The circuit also advantageously contains a flow-meter delivering the instantaneous flow value of the gas introduced. If it concerns liquid monomers which must be vaporized under the operating conditions within the reactor, they may be injected by means of a diaphragm pump or any other equivalent device. The injection circuit also comprises a flow-meter giving a direct reading of the instantaneous flow of the liquid supplied.
If it is desired to introduce monomers together so as to affect a copolymerization, the flow of each circuit may be controlled so as to obtain a precise and constant relative ratio between the monomers.
A gas-analyzer may generally be employed for at each instant controlling the flows in accordance with the desired reaction conditions.
Of course, also, the plant comprises boxes for controlling the temperatures and the flows and recorders for the various parameters of the reaction, such as:
temperature of the reactor,
temperature of the heat-carrying fluids in the different points of the plant,
agitation speed,
torque acting on the agitator,
reaction pressure,
flow of the monomer(s).
The evolution of the polymerization reactions may thus be followed in a precise manner.
The invention may be used to advantage for the prepolymerization of olefins, for example ethylene, propylene, 1-butene, and other olefins mentioned before.
The prepolymer may be prepared directly within the spherical polymerization zone so as to control the particle size of the catalyst and reduce its activity at the start of the reaction, after which the polymerization is continued in the same reactor under well-controlled conditions. But, preferably, the reactor of the invention is employed as an annex prepolymerization reactor, the prepolymer being then introduced in another polymerization reactor in a gaseous phase. Within the prepolymerization reactor, the catalyst grains grow and become less sensitive to the environment of the polymerization zone. The prepolymer having a catalytic activity is thereafter transferred into another reactor of any type having a fluid or agitated bed, or a spherical reactor according to the invention. The invention is perfectly suitable for producing such a prepolymer and the results obtained are as advantageous as they are surprising.